The present invention relates to ink jet printers, and, more particularly, to ink jet printers which utilize the so-called drop-on-demand method of operation.
Non-impact printers have recently become very popular due to their quiet operation resulting from the absence of mechanical printing elements impacting on record media during printing. Among such printers, ink jet printers are particularly important as they permit high speed recording on plain untreated paper.
Various ink jet printing methods have been developed over the past years. In the so-called continuous ink jet method, such as disclosed in U.S. Pat. No. 3,596,275, the ink is delivered under pressure to nozzles in a print head to produce a continuous jet of ink emitted through each nozzle. The ink jet is separated by vibration into a stream of droplets which are charged, and the flying droplets are either allowed to impact on a record medium or are electrostatically deflected for collection in a gutter for subsequent recirculation.
A second method, known as the electrostatic method, is disclosed, for example, in U.S. Pat. No. 3,060,429. In this method the ink in the nozzles is under zero pressure or low positive pressure, and the droplets are generated by electrostatic pull and caused to fly between two pairs of deflecting electrodes arranged to control the direction of flight of the droplets and their deposition in desired positions on the record medium.
A third method, which is known as the drop-on-demand method, is described, for example, in U.S. Pat. No. 4,125,845. The droplets in this method are emitted under the control of an electronic character generator by means of volume displacement brought about in an ink chamber or channel by means of energization of a piezoelectric element. The volume displacement generates a pressure wave which propagates to the nozzles causing the ejection of ink droplets.
The drop-on-demand method has several advantages over the other above-mentioned methods. Ink jet printers using this method have a simpler structure requiring neither deflecting means for controlling the flight of the droplets nor the provision of an ink recovery system. Multiple-nozzle print heads using this method are simple and compact and are relatively easy to manufacture.
The drop-on-demand method requires that under quiescent conditions there is an appropriate underpressure, i.e. negative pressure, in the ink chamber or reservoir, in order to retain the ink in the nozzle until such time that it is to be ejected. The amount of the underpressure is critical. With too small an underpressure, or with a positive pressure, ink tends to escape through the nozzles. On the other hand, with too high an underpressure, air may be sucked in through the nozzles under quiescent conditions. The required underpressure may be obtained gravitationally by lowering the ink reservoir so that the ink surface level therein is below the level of the nozzles. However, such positioning of the ink reservoir may not always be easily achieved, as it may require complex changes in the design of the ink jet printer or its print head. Moreover, it cannot be achieved in hand-held ink jet printers which must be capable of being tilted to print on a variety of objects such as parcels, packets, envelopes, sheets, or the like.
The above-mentioned U.S. Pat. No. 4,125,845 describes a pressure regulating system which maintains the pressure within predetermined limits by means of a piezoelectrically driven valve and a wire gauge pressure sensor and electronic control. This arrangement is complicated, costly and tends to malfunction because of its complexity. Also, it is disadvantageous to use electromechanically operated elements, such as valves, in a non-impact printer, since this will tend to reduce life expectancy as a result of wear of such elements.